Brake control system

ABSTRACT

A brake control system for a vehicle having a number of wheels with brake application device is provided with a slip error signal that is used to modify brake application means. The slip error signal is produced by comparing an instantaneous wheel speed signal to a simulated vehicle variable reference signal. The simulated vehicle variable reference signal is produced by a simulator supplied with wheel speed information obtained solely from one wheel to avoid error inducing introduction of wheel speed information from other wheels. An initial simulated vehicle variable reference signal is derived from maximum wheel speed attained during wheel acceleration. The simulated vehicle variable reference signal representing the speed of the vehicle during deceleration is produced by modifying the variable reference signal with a value including a force-analog signal representing the drag forces on the vehicle.

United States Patent 11 1 Hirzel Oct. 30, 1973 BRAKE CONTROL SYSTEM3,245,727 4/1966 Anderson et al 303 21 co Inventor: Edgar A. Hirul,Granada H 3,547,501 12/1970 Harned et al. 303/21 BE Calif. PrimaryExaminer-Milton Buchler [73] Asslgnee: Crime Company, Chlcago, AssistantExaminer-Stephen G. Kunin [22] Filed; June23, 1971 Attorney-James P.Hume et al.

[21] Appl. No.: 155,903

Related Us. Application Data [57] ABSTRACT [63] Continuation of Ser. No.871,439, Oct. 22, 1969, A brake 00mm! System for a vehicle having anumber abandoned, which is a cominuatiomimpan f Sen, of wheels withbrake application device is provided No. 665,072, Sept. 1, 1967,abandoned. with a slip error signal that is used to modify brakeapplication means. The slip error signal is produced by [52] US. Cl.303/21 BE, 188/181 C, 303/20, comparing an instantaneous wheel speedsignal to a [51] Int. Cl B60t 8/10 simulated vehicle variable referencesignal. The simu- [58] Field of Search 188/ 181 C; 244/111; latedvehicle variable reference signal is produced by 303/20, 21; 317/5;318/52; 340/263 a simulator supplied with wheel speed information ob-324/ 160-161 tained solely from one wheel to avoid error inducingintroduction of wheel speed information from other [56] References Citedwheels. An initial simulated vehicle variable reference UNITED STATESPATENTS signal is derived from maximum wheel speed attained 3 401 984[1968 Williams at 8] 303,21 BE during wheel acceleration. The simulatedvehicle vari- 3467443 9,1969 Okamoto at 303/2l- BE able reference signalrepresenting the speed of the ve- 3:498:682 3/1970 Mueller et al. 303/21BE hiele during deceleration is Produced by modifying the 3,614,172 101971 Riordan 303 21 BE variable reference Signal with a value includinga 3,604,760 9/1971 Atkins 303/21 BE UX force-analog signal representingthe drag forces on the 3,545,817 12/1970 Yarber 303/21 P vehicle.3,275,384 9/1966 l-lirzel....-. 303/21 CG 3,652,133 3/1972 Yamazaki etal. 303/21 CG 48 Claims, 8 Drawing Figures TRANSDUCER 7 Hm 7 05c. FEE[09 604/770! P5. & 5026. R56. cc7: 3- L 5 f 7 Ao' /0/ m I/EL, m ac I M M//7 a //5 //9 f fify/ A5 mil 5% r DRIVER Patented Oct. 30, 1973 6Sheets-Sheet l Patented Oct. 30, 1973 e Sheets-Sheet Patented Oct. 30,1973 6 Sheets-Sheet I Patented Oct. 30, 1973 6 Sheets-Shet 6 BRAKECONTROL SYSTEM This application is a continuation of my application forUS. Letters Pat., Ser. No. 871,439, filed Oct. 22, 1969 (now abandoned),which in turn is a continuation-in-part of my application for US.Letters Pat, Ser. No. 665,072, filed Sept. 1, .1967 (now abandoned),entitled Braking Control System for Aircraft Vehicles and the Like.

This invention is directed to a multi-wheel brake control systemsuitable for use on a vehicle equipped with independently brakablewheels. Each wheel activates v transducer and converter means fortranslating wheel rotationalspeed to electrical energy having an analogvalue proportional to wheel speed. The control system features meansproviding an electrical reference representative of and simulating theground speed of the vehicle together with means for producing,relative'to each wheel, an electrical value representing theinstantaneous wheel speed and comparator means having a dual output forcomparing this electrical value with the electrical reference simulatingthe vehicle ground speed. One of the dual outputs of the comparatormeans results from a minor error signal representative of incipientwheel skid and a small disparity between the compared electrical valuesand a first predetermined quantity, and is utilized to effect minorrelaxation of braking on the associated wheel. The second output of thecomparator means results from a gross error or an excess decelerationsignal representative of gross disparity between the compared electricalvalues and a second predetermined quantity, and is utilized to furtheraugment the brake relaxing action of the first one of the outputsignals, it being understood that the two output signal processing meansoperate in parallel independently of one another and additively. In thismanner the invention system operates during deceleration to provide areference signal simulating vehicle ground speed and including means forcontinuously generating at each braked wheel an analog signalrepresenting instantaneous rotational speed and for comparing thissignal with the vehicle speed reference. If the difference is small,relatively light brake relaxation. occurs, but if the difference islarge, substantially greater relaxation occurs. In addition, the. systemfeatures means operating at times to provide full. brake release untilthe wheels approach or resume a speed corresponding to the vehicleground speed before permitting further braking.

A paired wheel crossover circuit interconnecting the brake controls ofpaired wheels located along the opposite sides of the vehicle isresponsive to a gross disparity between the rotational speeds of thepaired wheels to release braking of the slow wheel as well as to providefail safe protection should the speed sensing circuit of either wheelfail. In this case, braking of the wheel having the unserviceable speedsensor is deactivated.

Another important feature is the provision of hydraulic lag compensationincluding separate means for compensating for the different lag valuesassociated with brake release and with reapplication of braking. Each ofthese'lag values is simulated and appropriate correction signals arecombined with the braking signals associated with brake relaxation andwith restoration of braking.

In the prior art, vehicle braking systems are known in which analogsignals are generated which represent the rate of rotationaldeceleration of a wheel being braked, and in which a reference signal isproduced which represents an experimentally determined maximumpermissible rate of rotational deceleration of the wheel, the twosignals being effectively placed in opposition whereby brake-controllingmeans, such as a valve, are quiescent or unenergized until such time asthe reference signal is exceeded by the wheel deceleration signal,whereupon the brake is released in proportion to the signal differences.Other of these prior systems utilize means for draining or bleeding awaythe wheeldeceleration signal via drain means to correspond with theamount of the wheel-deceleration signal produced when wheel-skidding issubstantially incipient. When the signal drainage capacity is exceeded,the excess signal value is used to initiate operation ofbrakecontrolling means to reduce the braking effort.

The present system presents certain improvements over the prior art suchas the provision of a continuously variable reference signalrepresenting the immediately previous wheel speed as modified by asignal representing what the entire vehicle braking system is currentlydoing. The standard of reference is the potential of a charge stored ina capacitive device and representative of craft speed, the charge beingcaused to decrease in proportion to braking effectiveness. This storedcharge is subject to variable drainage dependent upon the magnitude ofbrake-control signals furnished to the several brakes. The systemutilizes the reference potential and an analog potential representingthe instantaneous rotational speed of a wheel in comparator meansoperating to provide first and second output signals which are processedto provide separate brakecontrol signals if either output signal exceedsa respective threshold value. The first output signal is used to producea relatively moderate brake relaxation signal provided by the effects ofthe first signal up to a selected level of relaxation. The second errorsignal, augmented by the first, is used to relax the brake further up toand including complete release of the brake of the associated wheel,depending upon the disparity be- .tween the wheel velocity analog signalrepresenting the instantaneous rotational speed of the wheel and theinstantaneous value of the variable reference analog sig nalrepresentative of the craft ground speed.

It will therefore be understood that the present invention provides acomparator unit operable to provide separate minor and major errorsignals for each wheel to the end that these separate signals may beanalyzed separately of one'another and compared with respectivethreshold values of selected measure and then utilized alone and incombination to control braking action. Minor output signals, herein alsotermed PBM or pressure bias modulation signals, provide a primarycontrol and establish a variable mean pressure level of braking pressurefor normal braking conditions. The major or transient error signals areprocessed separately from the minor error signals and provide a majorcorrective signal to vary the mean braking pressure level in response tosudden large increases in wheel deceleration. Since these two errorsignals are processed in parallel circuits and separately from oneanother, each processing circuit can have its own threshold level, gainand limit values. For example, the PBM signal control can be set torespond to a limited range of small error signals whereas the transientsignal control can be designed to be relatively insensitive until theerror signals approach and exceed the end of the FEM control range.Under these circumstances, the transient control means generates ahigh-speed high-gain correction signal to reduce braking pressuresharply and, upon wheel spinup, return control to the FEM control unit.This arrangement is found to provide smooth variation in the valvesignal for normal braking operations and can be supplemented, on need,by a relatively large transient signal should the wheel encounter a wetspot, an ice patch or the like.

The present invention features a unique mode of establishing a referencepotential representative of the actual rotational speed for each wheel,This reference potential is tailored to the size of the particular wheeland to the tolerances of the components used in its own speed sensingcircuit. The reference potential for each wheel is established instantlyupon initial wheel spin up and is reestablished at new maximum butprogressively lower values each time the brake for that wheel isreleased and the wheel spins up. These maximum values may be the same ordifferent from the corresponding reference potentials of the otherwheels because of pos sible differences between the respective wheeldiameters and tolerance variations in the components of the respectivewheel speed sensing circuits. Each reference potential decays at a ratewhich corresponds to the maximum permissible vehicle deceleration rateunder optimum'runway braking conditions as modified by brake releasesignals from all wheels subject to brake control.

Another significant improvement in the present braking system arisesfrom the fact that instantaneous wheel braking is modified in accordancewith the performance of the braking system as a whole, that is, by avalue representative of the instantaneous ground speed of the craft.This expedient makes possible the same control sensitivity under a widerange of runway conditions over the entire range of vehicle landingspeed from touch-down to taxiing speed, a capability not achievable withprior anti-skid systems.

Under dry runway conditions, the craft is capable of decelerating at ahigh rate and with the relatively small PBM brake signals applied to thevalve to relax braking pressure as needed. Under these conditions thepotential representing craft velocity decays at a relatively high rateas the craft decelerates rapidly. Under wet, icy or low mu runwayconditions the brake valves will operate at relatively high correctionsignals with the result that the craft speed reference potential iscaused to decay at a slower rate. Thus the instantaneous wheel velocityis compared with a realistic aircraft velocity and an error signal ofgreater intensity is generated and utilized to provide an expeditedvalve control signal, as is desirable for wet or icy runway conditions.

Another feature of the present braking system is the fact that thetransient control signal is added to or summed with the FEM controlsignal to provide expedited and greater brake relief when a wheelstrikes a wet or icy spot, or a patch of smooth macadam. As soon as thewheel passes the low mu runway condition, the relatively severetransient error signal ceases and control is returned to the relativelysmooth and even level error control provided by the FEM unit. It istherefore seen that the present system has a highly flexible andadaptive capability for providing highly efficient braking under widerange variable runway conditions.

An important safety feature of the invention is provided by a pairedwheel crossover circuit coupling control circuits of two wheels atopposite sides of the vehicle together to safeguard against thepossibility of either wheel operating with brakes applied when theassociated wheel is locked or grossly over-braked. In addition thecrossover circuit provides fail safe operation in the event the speedsensor circuit for either of paired wheels fails. In this case, thebrake controlled by a nonfunctioning sensor circuit is renderedinoperative until the unserviceable circuit has been repaired.

Another novel feature of the invention concerns means for simulatingfluid inertia or time response characteristics of the hydraulic brakingsystem and operating to compensate for the delay in relaxing the brakesupon actuation of the control valve as well as for the delay inreapplying the brake when the valve is actuated in the oppositedirection. It will therefore be understood that the signal to theantiskid control valve employed to govern the brake of each wheel ismoditied to compensate for fluid inertia factors associated with bothbrake release and reapplication of the brake.

Accordingly, it is a primary object of the present invention to providean improved and more efficient braking control system featuring highsensitivity and more flexible responsiveness to ground conditions.

Another object of the invention is the provision of a braking controlsystem for aircraft and vehicles generally utilizing means for producingsignals representative of the changing ground speed of the craft andmodifying the braking action on individual wheels in a degree dependentupon the difference between the craft speed signal and the instantaneousspeed signal of each wheel.

Another object of the invention is the provision of a braking controlsystem for vehicles featuring means for establishing separatedeceleration reference signals for each wheel for comparison with asignal representative of the instantaneous rotational speed of theassociated wheel and including means for modifying each reference signalby a correction factor produced by the brake control signals of thevarious wheels of the system.

Another object of the invention is the provision of a highly versatile,quickly responsive braking control system having two means forseparately processing different types of instantaneous wheel speed errorsignals and utilizing the output of one processing means to provide arange of minor brake control signals and the output of the second meansfor providing a range of major brake control signals when more severecorrective action is needed.

Another object of the invention is the provision of a braking controlsystem for vehicles having means for providing separate wheel speeddeceleration signals and for separately processing signals relative toindivid ual predetermined threshold values and for utilizing theresultant outputs to control wheel braking.

Another object of the invention is the provision of a braking controlsystem for a vehicle having a plurality of wheels and utilizing brakecontrol signals from all the wheels to provide a reference representingthe ground speed of the vehicle for comparison with the instantaneousspeed of individual wheels to provide means for braking individualwheels as necessary for maximum braking effectiveness under existingroadway conditions.

Another object of the invention is the provision of a braking controlsystem for a plurality of vehicle wheels in which braking is modified bysignals obtained through instantaneous comparison of a wheel speedanalog signal with a variable analog signal representative of thecontemporaneous ground speed of the vehicle.

Another object of the invention is the provision of a braking controlsystem having improved means for assuming wheel spin-up beforepermitting application of the brakes.

Another object of the invention is the provision of a braking controlsystem having means associated with pairs of wheels positioned one oneither side of the craft and operating to sense wide scale disparitiesin their rotational speeds and responsive to such conditions to releasethe brake of the slow wheel pending spin-up.

Another object of the invention is the provision of a braking controlsystem having means for normally controlling the brake for each wheelindependently of one another and featuring fail-safe control meansinterconnecting the wheel circuits of each pair and operable todeactivate the wheel brakes of the inoperative wheel upon malfunctioningof speed sensor circuit.

Another object of the invention is the provision of a braking controlsystem for a multiwheeled vehicle having the brake control means forwheels on opposite sides thereof interconnected with one another andeffective, upon sensing a wheel speed disparity of predeterminedmagnitude, to release the brake for the slower wheel until the pairedwheels are operating at generally similar speeds. I

Another object of the invention is the provision of a braking controlsystem utilizing hydraulic braking and featuring means for compensatingfor time required to drain fluid from brake components when relaxing thebrake.

Another object of the invention is the provision of an anti-skid brakingsystem utilizing hydraulic braking and featuring means for compensatingfor time required to restore hydraulic pressure in the brakingcomponents when reapplying the brake.

Another object of the invention is the provision of an anti-skid brakingsystem having separate selectively operable hydraulic lag simulators forcompensating for fluid flow delays in releasing and reapplying the brakein response to a skid signal.

Another object of the invention is to provide'rneans for producing asignal representative of instant. rotational speed of a wheel and meansfor deriving from the produced signal a second signal representative ofground velocity of the vehicle, whereby disparity between such producedand second signals representative of excessive rotational decelerationof the wheel relative to vehicle speed may be sensed and utilizedtoinitiate relaxation of braking effort at the wheel.

These and other more specific objects will appear upon-reading thefollowing specification and claims and upon considering in connectiontherewith the attached drawings to which they relate.

Referring now to the drawings in which a preferred embodiment of theinvention is illustrated.

FIG. 1 is a schematic functional block diagram of the principal units ofan illustrative embodiment of the invention;

in FIGS. 3, 3a and 3b; and

FIG. 5 is a schematic block diagram useful in explaining functionalunits of the. circuits depicted more in detail in FIGS. 3, 3a and 3b.

IN GENERAL Referring to the drawings, thereis shown an exemplaryembodiment of the invention arranged for use with aircraft landingwheels, although it will be understood that the principles of theinvention are applicable to means for braking vehicles generally. Theshowing in'the drawings is directed more particularly to the componentsemployed to control a single Wheel and includes an indication of thenecessary connections between the similarly connected control componentsassociated with each of the other wheels. The invention system includesfor each wheel a suitable a.c. signal transducer, such as a conventionala.c. generator, driven by the Wheel to supply an a.c. signal thefrequency of which is directly proportional to the rotational speed ofthe associated wheel. The brake for each wheel includes any suitablebrake applying and releasing means well known in this art and preferablyof the hydraulic type subject to control by the pilot or other operatorand including an electrically actuated valve for releasing pressurizedfluid to a fluid return line when the invnetion control system signalsfor brake relaxation. The ac. wave produced by a wheel speed transduceris supplied to a squaring unit 10 to produce a substantially square-waveoutput signal having a frequency directly proportional to the rotationalspeed of the wheel driving the transducer. The square-wave output ofunit10 is fed to a frequency-to-d.c.-potential analog converter unit 20which has two output signals each havingapotential magnitudeproportional to the frequency of the square wave input signal. Eachcomprises an instantaneous wheel speed signal in the form of anintegrated variable potential, and one is supplied to comparator unit 30and the other is supplied to reference signal unit 40.

Comparator 30 is effective to compare the primary input signal from unit20 with a reference signal pro-- vided from reference signal unit 40 tobe described presently. When any wheel is braked to decelerate in excessof a determined rate, due to excessive braking force or an insufficientground co-efficient, comparator unit 30 detects this condition bycomparing the change in magnitude of the output signal from unit 20 withthe magnitude of a varying reference potential produced by referencesignal unit 40 and representative of the instantaneous ground speed ofthe craft. Comparator unit 30 has first and second output signal lineswhich may or may not carry an effective output signal depending on thenatures of the input signals to unit 30. The first output signal line isconnected to a PBM or small error control unit 50 and the other to atransient or gross error control unit 70.

If the input signal to PBM unit 50 is within preselected range ofmagnitudes above a determined threshold value representative of the needfor minor braking corrections, an output signal of appropriate magnitudeis channeled to driver unit 60 for the brake control valve. If there isa gross speed error signal in excess of a predetermined and separatethreshold value, then transient control 70 provides an intense outputsignal to driver unit 60 augmenting the signal then being received fromPBM unit 50 with the result that driver unit 60 provides a large brakerelaxing signal until the associated wheel spins up to provide anullifying signal. The extent of brake relaxation is proportional to theinput signals received by driver unit 60 from units 50 and 70.

The output signal of driver unit 60 is also applied, in common withsimilar signals supplied by each of corresponding driver unitsassociated with other of the vehicle wheels, to reference signal unit40. Unit 40 is arranged to utilize input signals supplied, respectively,by converter unit and a touchdown protection unit 90. The latter unit iseffective to provide a full strength brake-release signal to controlunit 80 until such time as the associated wheel is fully load bearingand rotating at full speed. Unit 90 is supplied electric power by anoleo switch which remains closed until the wheel becomes loadbearing.Other details of the exemplary system and its operation are describedbelow.

THE SYSTEM DETAILS Direct current power at 18 volts is supplied to thecircuitry from a closely regulated power supply via the positivelyenergized bus P and ground bus G. It will be understood that the variouscircuit components are interconnected as illustrated, and that R denotesresistor, C denotes capacitor, CR denotes diode rectifier, J denotes ajunction, and 0 denotes a transistor. Suitable exemplary values of thecomponents are listed in Table. l, below.

An a.c. wave signal from a wheel-actuated transducer is introduced tothe circuit on line T and is translated in known manner into a squarewave signal by unit 10 which includes a wave-shaping amplifiertransistor Q10. The latter amplifies the low level a.c. input signal,and a Zener diode CR 13 clips the output of Q10 to form a square wavehaving a constant amplitude and the same frequency as the transducersignal. As is evident, the input a.c. wave signal is applied to the baseof Q10 via resistor R4 and coupling capacitor C3 and the resultingsquare wave signal, generated across the circuit including resistor R25,appears at junction J1.

The square wave output signal of the squaring unit is translated by thenetwork including capacitors C4 and C5, rectifiers CR14 and CR15,resistor R26 and the input portion of transistor Q11, into an analogd.c. signal having d.c. potential level proportional to wheel rotationalspeed. The dc signal, so produced and appearing across capacitor C5, isinverted and amplified by means comprising transistors Q11 and Q12, theamplification being at a fixed gain determined by the ratio of thevalues of resistors R27 and R26. The signal so produced appears at J9and has an analog potential varying directly with rotational wheelspeed. The circuit component values are so chosen that when thetranducer input frequency is zero, Q11 is maintained in saturation bythe normal current through R27 and the voltage developed across resistorR29. In this manner, the quiescent voltage developed across R29establishes a low speed velocity threshold or drop-out point for theoutput signal from amplifier Q11Q12.

The latter amplifier accordingly remains inactive until the wheelrotational speed provides a square wave signal producing a signalcurrent through R26 equal to the saturation current directed through R27by the threshold voltage apparent across R29. Accordingly, at low wheelspeeds amplifier Q11Q12 is inactive and, for reasons explained morefully presently, the wheel speed signal is incapable of interfering withmanually controlled braking operations during taxing and parkingoperations.

Comparator unit 30 comprises a differential amplifier circuit featuringamplifiers Q3 and Q4 the currents through which are restricted to a sumvalue held constant by constant'current transistor Q14, the biaspotential of which is fixed by resistors R35, R36 and R13 and rectifierCRIS. The wheel speed analog potential apparent at junction J9 isimposed, via respective rectifiers CR3 and CR2, onto the base electrodesof each of amplifiers Q3 and Q4. This same wheel speed potential is alsoemployed to charge the vehicle ground speed reference capacitor C8.Decay of the latter charge by discharge via Q15 and R37 is regulated ata rate representative of the desired instantaneous maximum permissibledeceleration rate for the vehicle.

When, the potential at J9 representing instantaneous rotational speed ofthe wheel decreases at a rate faster than the vehicle speed referencepotential on C8, rectifier CR2 becomes back biased, 04 remains biased bythe potential of C8, and the bias of Q3 decreases with the decreasingwheel speed potential. This condition indicates the wheel isdecelerating excessively and, in consequence, Q4 conducts increasingly,and Q3 conducts decreasingly, because Q14 operates to hold the sum ofthe currents through Q3 and Q4 constant. As a result of thisdifferential action of Q3-Q4, a positivegoing potential signal iscreated at junction J21 and concurrently a negative-going potentialsignal is created at junction J22.

Thus there is generated at J21 a PBM wheel velocity error signal whenand only when the potential drop across R9, CR4 decreases to a value atwhich Q3 is biased toward the nonconductive state. Similarly, there isgenerated at J22 a transient wheel velocity error signal when and onlywhen the potential drop across R12-CR5 increases to a value at'which Q5is biased to the conductive state.

The positive-going signal at J21 constitutes a first output signal ofcomparator unit 30 and this signal is applied to the base of transistorQ8 of the pressure bias modulated or PBM control unit 50. Thenegativegoing signal appearing at J22 constitutes a second output signalherein called a transient control signal, and this signal is applied tothe base of transistor Q5 of transient control unit 70. The two notedsignals, as heretofore indicated, may be appropriately designated -errorsignals. If these signals are small, they indicate wheel deceleration ata rate moderately faster than the rate of deceleration of the craftitself as represented by the rate of discharge of the variable referencecapacitor C8. A stable condition of the differential amplifier Q3-Q4exists during wheel spin-up as the aircraft touches down and wheneverthe wheel speed corresponds with the ground speed of the craft lesspermissi ble creep. An unbalanced condition exists whenever the wheel isbeing braked so as to have a rotational speed less than that equivalentto ground speed of the craft minus the permissible non-skidding creep ofthe wheel tread relative to the track. This condition is evidenced by asmaller potential at J9 than on capacitor C8.

The circuitry of unit 40 comprises chiefly reference capacitor C8transistor Q controlling the discharge of C8 and control feedback loopsfrom the valve drive circuit of all wheels and which loops include arespective one of the resistors R50, R51, R52 and R53. For example, itwill be understood that R53 is in the feedback loop connected to theoutput of the driver circuit for the particular wheel circuit shown inFIGS. 2a, 2b. Similar currents are supplied to J50 by similarconnections to similar driver units associated with each of the otherwheels. The currents thus supplied via R50, R52, and R53 to J50 arethere added and flow to ground G via resistor R37, which is the sameresistor through which reference capacitor C8 must discharge viatransistor Q15.

It is, therefore, evident that the effective bias on Q15 and the decayrate permitted for C8 is dependent upon the summed currents through J50.As previously noted, the reference potential level at C8 relative to thepotential at J9, determines the balance or degree of unbalance ofdifferential amplifier 03-04 and depends upon the rate at which C8discharges. The instantaneous rate of decrease of the referencepotential on C8 is thus determined by the contemporaneous rate at whichC8 discharges through Q15. The rate control feed back via R50, 51, 52and 53 will therefore be understood as effective to adjust theinstantaneous Q15 collector current to discharge C8 at a rate closelysimulating the contemporaneous instantaneous vehicle deceleration.

The current discharging from capacitor C8 via Q15 dictates the rate ofcharge of the reference potentialof C8. Thus if the current iscontrolled by means of the circuit parameters of Q15 base and thecurrent at J50 in a manner accurately representing vehicle deceleration,the rate ofchange of the reference velocity potential of C8 will be thesame as the rate of change of aircraft velocity. Thus the wheel velocitysignal will be compared to an accurate representation of the aircraftvelocity as normalized for circuit component tolerances, tire diameter,etc. I

An alternative method and means for providing a representation of theground speed of the vehicle utilizes a free running wheel and generatormeans driven by the wheel and generating a current of intensityproportional to the rate of change of ground speed of the wheel, thegenerated current being applied to the discharge circuit for capacitorC8 provided by Q15 and R37. Still another mode of enabling C8 tofunction in providing a reference potential representative of the actualground speed of the craft or vehicle at any time may comprise suitabletransducer means, as for exampic at potentiometric accelerometer,operating to introular vehicle. As is evident from the drawings, therate control feedback loops provide feedback currents proportional tothe brake control signals produced by the respective driver units, suchas unit 60. The sum of these several feedback currents is' a function ofthe total instantaneous change of braking effort from that required toproduce maximum permissible deceleration under ideal conditions andtherefore simulates the change of actual ground speed under existingrunway conditions.

Returning now to the noted error signals created at J21 and J22 by theunbalance of O3-Q4, it will be understood that boththe PBM and thetransient wheel speed error signals apparent respectively at Q8 and OSare proportional to the velocity error, i.e., to the disparity betweenthe reference signal at C8 and the velocity signal at J9. The transientor major error signal, applied to Q5 from J22, is amplified by Q5 andprovides relatively large brake control signals. For Q5 to become activeto initiate a brake controlling action, the error signal from J22 mustbe sufficient to overcome the departure threshold bias potentialprovided at the emitter of Q5 by the voltage divider comprising R14 andR39. When that occurs, Q5 produces an output signal at J30 which isapplied to the unity-gain'amplifier Q6-Q7 of driver unit 60. As will bemade evident below, the output signal of O5 is summed at J20, J30 withthe output signal from PBM control unit and these combined signals areutilized to relax the wheel brake.

ThePBM control circuit unit 50, comprising transistor Q8, receives thepositive-going minor error signal from comparator unit 30. This signaldrives Q8 toward cutoff and, in doing so, causes the voltage drop acrossR42 to fall, thereby bringing the potential at J34 more negative andcausing O9 to conduct as soon as the R42 voltage drop falls below adeparture threshold level determined by the voltage division of R20,CR20 and R44. A voltage clamp provided'by R19, R43, and CR8 clamps thevoltage drop developed across R20, which in turn clamps the collectorcurrentof Q9 when the error signal reaches a predetermined upper level.When the voltage drop across R20 is intermediate the departure thresholdlevel and the clamped potential level, the current conducted by Q9 isproportional to the magnitude of the error signal. The current'passed byamplifier O9 is integrated with respect to time by charge path forcapacitor C10. Thus Q17, in effect,

provides a fixed negative going error voltage when it is integrated byC10 along with the positive going error signal from Q9 mentioned above.The resultant integrated error voltage across C10 is coupled to driverunit 60 by amplifiers Q16 and Q18, and is summed at J20-J30 with thetransient error signal provided by Q5. The summed error signal, apparentat J30, is the input signal to the unity gain amplifier 06-07 of driverunit The-output'of Q6-.-Q7 is connected to the valve via CR23 andoperates-brake control unit to relax the brake. This output is alsoconnected throughRSS to junction J50 to contribute to the feedbackcontrolling the craft speed reference voltage at C8.

Under reasonably uniform runway conditions, there is a correspondingoptimum deceleration for the vehicle and a corresponding braking effort.For any optimum deceleration the PBM reference capacitor C10 remainssteady with the discharge rate via Q17, R46 equaling the charging ratevia Q9, R20. Any deviation from this steady condition is the result ofan increase or a decrease in the error signal. The error signal is thedifference between the instantaneous wheel speed signal and the craftvelocity reference'signal. If the reference velocity deceleration is anaccurate representation of the vehicle deceleration, then there isprovided an.accurate error signal for use in modifying the PBM signaland which in turn is an accurate measure of the required relaxation ofbraking effort. Since the optimum braking effort corresponds to anoptimum deceleration of the vehicle, the PBM signal can be used toaccurately control the deceleration reference velocity signal. In thismanner there is provided a closed loop between comparator unit 30, PBMcontrol unit 50, driver unit 60 and reference signal unit 40establishing optimum operating error velocity for the wheel for optimumbraking effort and optimum slip.

If braking pressure increases sufficiently to reach or pass the optimumslip point, the associated error voltage integrated at C10 rises. Thatvoltage acting through 016-018 on driver unit 60 prolongs'brakerelaxation until equilibrium is attained. Under equilibrium conditions,the error signal current from the collector of Q9 equals the constantcurrent drain of Q17, C10 ceases to charge, and the wheel slip ismaintained at optimum level for maximum effective braking.

Any change in the wheel runway conditions from those; representingoptimum braking conditions decreases the slip from the optimum value,and a corre-.

sponding change occurs in the integrated error voltage across C10 untilthe resulting brake-control signal restores the slip to its optimumvalue. Hence, for small variations in wheel-track coefficientencountered during intensive braking, the brake-control signal will seeka constant level at or near the optimum level for most effectivebraking. However, when large-scale differences in wheel-trackcoefficient are encountered, as on I a runway having spots of ice, thetransient error signal is likely to predominate and brake control isderived largely via transient control amplifier O5 to relieve brakingfast and to a greater extent.

It will be understood that among the functions of certain components,not fully described above, are the following additional functions. Thus,diodes CR27 and CR28 in the circuit containing junctions J20, J30, atwhich the PBM and transient control signals are summed, providetemperature compensation for the base-emitter voltage drop of Q6 andcoupling diodes CR22, CR23, R17, R41 and CR7 provide a voltage clamp tolimit the maximum excursion of the brake control signal voltage toprevent overdrive of the brake relaxing control. It is also pointed outthat diode CR7 is normally reversed biased until the base potential atO6 reaches a maximum value at which diode CR7 becomes forward biased andlimits excessive increase of the brake control signal.

BRAKE LOCKOUT DURING TO UCHDOWN.

Positive means for assuring lockout of the braking during aircrafttouchdown on a runway is assured by the invention antiskid control bymeans of the circuit including R49, CRl6, Q13, R33, R34 and CR17operating in conjunction with differential amplifier Q3-Q4. The functionof this circuit is to assure complete brake release prior to touchdownof the craft on the runway and spin-up of the wheels. For this purposethere is provided a squat signal, this signal being supplied via a squatswitch closed so long as the aircraft is airborne, and opening as thecraft touches down on the runway.

A strong squat signal current is supplied through the squat switch froma 28-volt supply and through R49 and flows in part through R33, R34 toground. The sig nal thus produced at J48 is supplied directly to C9 tocharge this capacitor, and via CR17 to capacitor C8 and the base ofdifferential amplifier Q4. The strong signal applied to Q4 provides astrong continuous error signal at J22 which, via transistor Q5, anddriver Q6, Q7, produces a very strongbrake signal to fully release thebrakes.

Following spin-up, the voltage drop across R30, R31 increases due to thegeneration of a wheel signal. The resultant potential at R30, R31 causesQ13 to conduct, thereby causing C9 to discharge via R33, Q13 and R32 andvia a parallel path through R34. This also removes the source of chargeon C8 and the base of Q4 except as providedlater via J9 by'the spin-upof the wheel. Thus, C9 quickly discharges causing Q17 to conduct heavilythereby insuring the discharge of C10 and reducing the PBM to zero.

Incident to landing, the squat switch opens, thereby removing the sourceof potential at J44 and J48 and returning full control to the potentialat C8 as determined by J9.

As is evident, prior to touchdown the brakes are held fully relaxed andthe PBM potential stored inClO is at a maximum. As wheel spin-up occurs,this charge is dissipated through Q17 via C9 in the manner justdescribed, allowing the brake control signal to decrease to zeroallowing the brake to apply rapidly but smoothly.

The crossover circuit includes CR10 and R5 of each wheel circuit. Theseseries connected components are connected between the junction of thebase of Q4 and reference capacitor C8 of one wheel control circuit, andthe junction of R29, R30 of the other wheel control circuit in themanner indicated in FIG. 2a.-So long as both of the paired wheels arebeing braked normally, the rotational speed potentials are closelyrelated and CR10 of each crossover circuit is back biased by the wheelspeed signal of the respective wheel circuits thereby decoupling thepaired wheel speed potentials of each wheel from the differentialamplifiers 03-04 of the other wheel. However, should the relative speedsof the paired wheels differ by a preselected value, as 60 knots, thevoltage drop across R30-R31 of the wheel whose circuit is normal orwhich is at the higher velocity remains at a level substantially abovethe potential at J9 of the non-functioning or lower velocity wheelcircuit. The resultant potential supplied by the properly functioningcircuit will therefore cause CR10 of the non-functioning or low velocityspeed sensing circuit to conduct, supplying a potential at the base ofQ4 much higher than the potential at the base of Q3. As a result thenon-functioning circuit will exhibit a large unbalance in the comparatoramplifier and signals will be generated to release the brake until suchtime as the circuit regains a potential at J9 corresponding to normalwheel velocity. During this interval the properly functioning circuitoperates in a normal manner.

It will therefore be apparent that the paired wheel crossover circuitserves additionally as a fail-safe control to insure the brake releaseshould the wheel speed signal producing circuit fail either prior to orduring a braking run. In the event of such failure, the speedrepresentative potential at .19 of the nonfunctioning speed potentialgenerating circuit disappears and CR for that wheel becomes forwardbiased from the properly' COMPENSATION FOR HYDRAULIC SYSTEM INERTIA Highspeed-and accurate control of maximum effective braking under wide-rangeroadway conditions using hydraulically actuated brakes necessitatesprovision for anticipating and compensating for factors of I hydrauliclag. Thus there is a time lag between a signal calling for a change inthe braking fluid pressure and the time the desired change in breakingbecomes effective, and irrespective of whether the change involves anincrease or a decrease in the fluid pressure. However,

' the two types of lag are substantially different in magnitude, thataccompanying brake release being much less than that associated withreapplication of braking.

To provide compensation for the foregoing, the invention system providesa hydraulic lead simulator comprising capacitors C13, C14, transistorQ19, diode CR34 and resistors R61, R67. This circuit receives its signalinput potential from the value driver circuit at the emitter of Q6. Thissignal is applied to the base of Q19 and, if sufficiently high, rendersQ19 conductive to provide a rate'current flow to charge capacitors C14.This rate current flow increases the potential drop across R12-CR5 toprovide a signal which is summed at the base of 05 with the initialtransient control signal from the differnetial amplifier to increaseconduction through O5 to cause 06-07 to produce an initial augmentedbrake relaxing valve signal. This increased valve signal expeditesoperation of the valve to relax brake application and thereby increasesthe speed of brake relaxation. As C14 charges, the current rate throughQ19 decreases and reduces the valve signal toward that actually calledfor by the differential amplifier.

The operation of the braking pressure recovery accelerator or augmentoris more complex. This accelerator as herein disclosed by way of exampleconsists basically of transistors Q20, Q21, Q22, resistors R62, R63,R64, capacitor C12 and diode CR29 and functions to provide a pressureboost signal of a nature causing the fluid pressure to rise inanticipation of further decrease of the valve signal during recoveryfrom low braking pressure. To this end the recovery simulator functionsto effect expedited closure of the valve during wheel spin-up therebycausing the brake cylinders to refill more quickly and therebyreestablish braking pressure at an accelerated pace. The increased fluidpressure is v 14 sensed across valve driver transistor Q7 of potentialthereacross.

I In operation, as the valve signal rises toward full brake releasevalue, the voltage differential between the emitter-collector of Q7 andrepresentative of brake fluid pressure, decreases substantially to zero.This differential drop causes O21, O22 to cease conducting and C12 todischarge through R62, 07 and R63, thereby simulating fluid dumping fromthe brake. While this is occurring it will be understood Q22 isnonconducting.

If the valve signal then decreases to initiate valve recovery andreapplication of the brake, the collectoremitter voltage differentialacross Q7 increases and C12 charges through R62, R63 and simulatescommencement of restoration of the fluid volume and pressure in thebrake components. During this period Q20 and Q21 remain cutoff until C12is partially recharged, but amplifier Q22 becomes conductive during therecharging period and produces a current flow through R64 and R56. Theincreasing voltage drop across R56 generates a simulated hydraulic erroror lag compensating signal, by-reducing the emitter potential on 018thereby reducing the signal to valve-driver Q6-Q7, and hastening cut-offof the driver and closure of the valve and reapplication of the brake.

As C12 becomes fully charged to simulate completion of the recoverycycle and full volume in the bra-kc actuator, Q20 and Q21 becomeconductive and the voltage drop across R63 falls to zero causing Q22 tocease conducting. There is then no longer a drain from Q18 emitter andbrake reapplication proceeds normally as the brake relaxation signaldecays to zero. At this time the hydraulic recovery simulator againfunctions in the manner just described to compensate for lag inrestoring fluid pressure and volume.

It will be understood that the above described hydraulic lag or inertiacompensating means are desirable optional auxiliaries enhancing theperformance of the principal system, but are not necessary to theoperability of the principal system.

Examplary values of the circuit components shown in FIGS. 2a and 2b areas follows:

by an increase TABLE I R4 I 470 ohms R34 9.lK R5 5.] K R35 2.IOK R7 l KR36 2.10K R8 6.8 K R37 7.5K R9 l5 K R38 36K R10 1.82K R39 60.4K R11l.82l( R40 6.2K R12 l5K R41 3K R13 34.8K R42 64.9K R14 ISK R43 2K R15200 ohms R44 28K R16 150K R45 36K R17 3.3K R46 1 10K R18 l5K R47 lOK R191.5K R48 15K R20 15K R49 3K R21 ISOK R50 430K R22 2K R51 430K R23 l.8KR52 430K R24 47K R53 430K R25 22K R54 6.8K R26 4.3K R55 1.3K R27 43.2KR56 15K R28 I30 ohms R57 15K R29 43.2 ohms R58 51 ohms R30 4.3K R59 20KR31 4.3K R60 47K R32 56 ohms R61 47K R33 15K R62 1.3K R63 36K O12 2N3l34CR30 IN645 R64 82K Q13 2N l 7ll CR32 IN645 R65 51 ohms O14 2Nl7l I CR33Gl30 R66 20K Q 2N l 711 CR34 IN645 R67 47K Q16 2N2605 CR35 IN645 Q172Nl7ll CR49 IN963B C2 0.47mfd Q18 2N2605 C3 Smfd Q19 2Nl7ll C4 0.0l2mfdQ 2N3 I34 C5 2.7mfd Q21 2N3l34 C6 0.22mfd Q22 2N 1 7H C7 22mfd C8 lOOmfdC9 22mfd CR8 G130 C10 lOmfd CR9 G l 30 C11 O.lmfd CRlO IN645 C12 47mfdCRll IN645 C13 0.0l5mfd CR12 IN645 C14 2.2mfd CR13 [N645 CR14 IN645 Q12N3|34 CRIS [N645 Q2 2Nl7ll CR16 lN963B Q3 2Nl 7ll CR17 ln645 Q4 2Nl7llCRIS G130 Q5 2N2605 CRl9 G130 Q6 2N l7ll CR20 IN645 Q7 2N3 l 34 CRZllN96l Q8 2N2605 CR23 ln965B Q9 2N2605 CR27 lN645 Q10 2Nl7ll CR28 Gl30Qll 2Nl7l l CR29 ln645 SECOND PREFERRED EMBODIMENT In FIGS. 3, 3a and 3bis depicted in schematic diagram form a modified circuit arrangementaccording to the invention. Therein the power supply unit, the regulatorunit, the transducer unit and the squaring circuit unit of the circuitrymay be conventional and similar to like units of the previouslydescribed circuitry, and are accordingly shown in block-diagram form.The transducer unit produces an electric wave output whose frequency isproportional to the assocaited wheel.

In FIGS. 3, 3a and 3b, the circuitry for and associated with one wheelis shown in detail, while that for the opposite wheel of a pair is, forconvenience and brevity, shown in block diagram form. The purpose andobject of the modified circuit arrangement is to optimize to the maximumextent possible the braking effectiveness of the system. While thecircuitry is, for reason of brevity, here restricted to that for twowheels, one at either side of the vehicle, it is evident that the systemis equally applicable to additional pairs of wheels.

As in usual braking systems, braking effort as evidenced by retardationof the vehicle is increased with increasing application of brakepressure until a point'is reached at which further application ofpressure causes excessive slip of the wheel tire tread relative to thesurface of the pavement, and the braking effort then decreases from amaximum value which is attained at the optimum tire slip point.Referring to FIG. 4, the graph shows variation of braking effort asbreak pressure is increased. The curve is that resulting from plottingbraking effort as ordinates and brake pressure as abscissae. A maximumis reached at point Q. The scale is arbitrary, and depends, inter alia,upon coefficient of friction, vehicle mass, and other variables. Themaximum, at Q, defines the optimum tire slip point.

In accord with the principles of the invention as exemplified in thesystem herein disclosed, optimum braking is attained by application ofincreasing brake pressure to a value M corresponding to that for point Qin FIG. 4, and at that point seeking to change the applied brakepressure as may be required to maintain braking effort at that point. Itwill be understood that the shape of the curve varies as the nature ofthe pavement surface varies, and hence the location of point Q shiftsboth as to ordinate and as to abscissa. Thus, in a hydraulic system, ifthe braking pressure has been increased to a value at which slipincreases beyond the optimum tire slip point value, the present systemoperates to reduce the applied pressure, and conversely,

when the slip decreases below the optimum tire slip point value, thesystem acts to increase the applied pressure. Thus the system firstseeks the maximum permissible brake pressure and braking effort,- andthen cuitry. That circuit action may be represented by the equation:

e,.= K I (e e,) d!

where e, is herein termed the control circuit velocity error signal, e,is a fixed threshold signal level, 2,- is the value of the voltageapplied to the valve coil, and K is aconstant.

From equation I it is evident that the valve signal will increase whene,e,'is greater than zero, and the rate of the increase is proportionalto the magnitude of the difference. Conversely, the valve signal willdecrease, and thereby permit the brake pressure to increase, when e,e,is less than zero, and again the rate of increase of pressure isproportional to the magnitude of the indicated difference. It is alsoevident that when e,-e, the valve voltage or signal will be constant andwill neither rise nor fall, and the brake pressure will be constant. Theparameters ofe and e, are indicated in FIG. 4. The following equationidentifies the relation between measured aircraft velocity v,,, measuredwheel velocity v,,., and e,, viz:

('11) noting that e, is a measure of the slip velocity. Any time theslip error exceeds e,, the signal to the integrating section of thecircuit is positive and, when integrated, causes reduction of theapplied brake pressure by initiating valve action. Conversely, any timethe slip error is less than e, the error signal is negative and permitsthe applied pressure to increase under the action of the brake pedal.

Thus it is evident that if e, is made to always correspond to the peakof the applied force/braking effort curve shown in FIG. 4, and e, is infact true slip between tire and pavement, then the system will reduceapplied pressure whenever the slip exceeds the peak value indicated atpoint 0, thereby reducing the brake torque and allowing the wheel toaccelerate and the slip value to increase back. to the peak value wherethe applied pressure becomes steady. If as a result of the pressurereduction the brake torque is reduced excessively, the slip will againbe less than optimum, with a value somewhere along the left limb of thegraph curve, the wheel speed or velocity error signal to the integratorwill be negative, and braking pressure will again be allowed to increasewith resultant increase of slip to the maximum value at e,.

The preceding operations are based upon the assumption that e, is suchas to coincide with the ordinate through the maximum braking effort oroptimum slip point Q. The circuitry permits e, to adjust to the valuecorresponding to that ordinate.

Considering equation II, it is noted that even if the actual aircraftvelocity, v,,., were known, at what value the threshold e, should be setto result in optimum slip would still be not known. The peak of thecurve could be at (v v 6 feet per second, or 8 feet per secnd, or othervalue. A set value for e, could be chosen and some measure of brakingefficiency attained if the measured values of aircraft and wheelvelocities v and v,,. were used, but the efficiency would be conjecturaland intermediate. And although it is possible to measure velocity of awheel, as distinguished from rotational speed, it is not feasible tomeasure two wheels, one braked and one not braked, with sufficientaccuracy to determine the slip velocity with the desired precision.

Considering again FlG. 4, although the actual location of point O, thatis,.the peak of the curve, is not known, it is nevertheless evident thatthe vehicle deceleration will be a maximum when the optimum wheel slipis at the peak value. Hence, if the rate of vehicle deceleration isknown, and is brought to a maximum, the peak value of braking effort hasbeen attained. Considering the equations of motion applicable to thewheel and the vehicle, the following are set down:

m dv /dt= F], F Fa (Ill) and I M dv ldr F, R,

wherein:

m mass equivalent of the wheel M mass of the vehicle F,,= brakeequivalent force F horizontal ground reaction force F summation of theother drag forces on vehicle v,= wheel velocity v,= vehicle velocityF,,= component of Fd acting on the wheel If, then, by choice, F,, 0, PO,

il m ri/ then m dv /dt F,, F F m/M Considering the case wherein brakingeffort F is such as to maintain the tire so that the ground reactionforce is a maximum, which is not the case when F =F,,, where F is themaximum value of F,,, it is noted that:

But dv ldt dv /dt since the tire and the vehicle decelerate at the samerate. Hence, from (V) and (VI):

Equation (VII) shows that the brake force F,, must be slightly greaterthan F ,,m for attainment of optimum deceleration of the vehicle. Notehowever that the unbalance force due to braking of the wheel is F,, F,,,so the magnitude of unbalance force that must be controlled by thecontrol system is F,,b F,, F where F,,b is the braking unbalance force.Hence at the mentioned peak,

Hence, the unbalance force which must be controlled by the controlsystem is equal to the ground reaction force multiplied by the ratio ofthe mass equivalent of the wheel to the mass of the vehicle. For typicalaircraft the masses and the relation to the ground reaction force F areas follows:

Aircraft Type Aircraft Mass Wheel Mass F l6 wheels 21200 slugs 6 slugs0.03 8 wheels 7300 slugs 6 slugs 0.08 4 wheels 3000 slugs 6 slugs 0.2

Thus it is evident thatthe control circuit must effect extremely precisecontrol to hold the braking at the optimum wheel slip value at the peakof the curve. (FIG. 4). The actual wheel velocity cannot conveniently beaccurately measured, and hence the relative slip between the tire andthe pavement cannot be accurately computed. However, referring toequations (lll) and (IV), supra, it is noted that the derivative of thevehicle velocity is related to the wheel velocity as indicated in thefollowing equations, in which v,,= slip velocity:

Hence, if the slip velocity is known and differentiated and summed withthe derivative of the wheel velocity, the result is the rate of changeof velocity of the vehicle.

'The circuitry depicted in schematic form and herein- .after describedin some detail-measures wheel velocity.

Prior to application of brakes, the maximum value of wheel velocity isthe vehicle velocity. That initial value of aircraft velocity is used tomeasure instantaneous slip velocity, that is, the difference betweenvehicle velocity and wheel velocity. As is dictated by the foregoinganalysis, the derivative of the slip velocity signal is determined, andadded to the derivative of the wheel velocity signal to give a signalrepresentative of the rate of change of vehicle velocity, which lattersignal is ,used to modify the computed vehicle velocity signal. Thelatter modified vehicle velocity signal is at any instant the I theoptimum slip value, that is when the operation is at the peak of thebraking effort versus slip curve. 7

' From the foregoing it will be recognized that if the wheel velocitysignal and the reference signal are coincident, there is no errorsignal, and therefore the brake unbalance forces on the wheel are atzero value. The wheel is then decelerating only as a result of dragforces at other wheels and/or aerodynamic drag and thrust reverseforces. Referring to FIG. 4, the lack of an error signal will result indecrease of brake valve signal and consequent increase of brakingpressure and braking effort. That increases deceleration of the wheel,and as wheel deceleration rises above vehicle deceleration the errorsignal increases and the rate of increase of brake pressure is reduced.Thus the system continuously seeks to keep the braking effort at thepeak of the curve and to maintain vehicle deceleration at maximum value.The slip velocity threshold e, in FIG. 4, is incorporated in the systemto demand an error signal such that the control loop continuously seeksthe maximum braking effort.

The modified system depicted in FIGS. 3, 3a and 3b is shown in moreabbreviated form as a functional block diagram in FIG. 5. The powersupply and regulator unit 101 supplies closely regulated d.c. power of,for example +l5 volts and +4 volts to the 13+ lead and to the transducerand 8+2 and sensor-return leads as shown in FIG. 3. The transducer 103at, for example, wheel 1 produces an output wave signal as indicated inFIG. 3 which signal is shaped and limited by the wave squaring circuit105 into a square wave the frequency of which is proportional to therotational speed of the wheel. The constant-amplitude square wave outputof the squaring circuit is transmitted to the velocity-tod.c.-converter107 whose function is to convert the variable-frequency square wavereceived from circuit 105 into a d.c. voltage signal whose amplitude isproportional to the frequency of the square wave and hence to. therotational speed of the wheel.

The d.c. signal produced by converter 107 is transmitted to each ofunits 109 and 111. The latter unit, comprising operational amplifier,hereinafter designated op-amp, units A6 and A6 of the circuitry in FIG.3, functions to determine the rate of change of reference velocity basedupon wheel velocity change and mass-ratio of the vehicle to wheel. Itacts to differentiate the low-frequency component of the incoming signalto provide a measure of rate of change of velocity. The output signalfrom unit 11 1 is transmitted to velocity reference signal unit 113which establishes a measure of instant vehicle velocity from the initialvelocity and the instant signal from unit 111. The initial velocity isthat represented by the signal immediately prior to braking.

The output signal from unit 113, representative of instant vehiclevelocity is fed back to unit 111 as indicated by the heavy line from theoutput of op amp 45 to the input of op amp A6 in FIGS. 3 and 3a; and isalso supplied to the velocity comparator unit 109. Unit 109 functions tocompare the wheel velocity as represented by the wheel speed signalproduced by unit 107, with the simulated vehicle velocity represented bythe output signal of unit 113, and to produce an output signal which isthe previously mentioned velocity error voltage or signal.

The latter signal is supplied to the op amp A8 of FIG. 3a by way of theheavy line signal path joining the output of op amp A4 (FIG. 3) andthepositive signal input terminal of A8. Op amp A8 is comprised in theaforementioned summing circuit which is included in the summingamplifier and transient control unit 115. The velocity error signal isalso transmitted to op amp A7 (FIG. 3) of the FEM control circuitcomprised in PBM control unit 117. The latter unit integrates errorsignal up to a limiting level, to provide pulse bias modulation controlvoltage signal effective to accommodate mild error signal levels toprovide moderate braking correction. The output of unit 117 is appliedto the negative signal input terminal of op amp A8 of the summingamplifier.

The summing amplifier and transient control unit 115 provides an outputsignal that is transmitted to the valve driver amplifier unit 1 19 whichcomprises transistors Q1, Q2 and Q3 (FIGS. 3a and 3b). As will beexplained in connection with the description of the circuitry depicted,unit 1 19 provides current to the coil of the servo valve unit 121 ininverse proportion to the required brake pressure reduction. Thus thegreater the current supplied, the more brake pressure is relieved orrelaxed.

The two wheels of a pair, here denoted wheel 1 and wheel 2, and each ofwhich is on a side of the vehicle opposite the other, are provided withsubstantially identical circuits, that for wheel 2 being condensed inFIGS. 3, 3a and 3b to block diagram form except as is hereinafter noted.Description of details will thus largely be restricted to the circuitryassociated with wheel 1 only.

Referring now to the detailed circuitry schematically portrayed in FIGS.3, 3a and 3b, the units denoted 101, 103 and 105 are or may be like theunits of the first embodiment herein described, or may be of othersuitable construction. Preferably, unit 105 comprises means to attenuateany high frequency voltage which may be present as electrical noise inthe transducer or sensor leads. The output of the squaring circuit maybe, forexample, clamped at 8 volts on the negative-going portion of thecycle, with the square wave signal going to zero volts during thepositive half-waves of the transducer signal.

The input circuitry connected to the signal input terminals of op amp A3is a frequency-doubling discriminator circuit. Capacitor C1 charges anddischarges alternately with successive halflcycles of the input signalsquare wave. A resultant charge transfer takes place between capacitorsC2, C3 and C4, causing the voltage at the R2-C3 junction to increasewith frequency, and that at the Rl-CZ junction to decrease withfrequency of the square wave. The output network R5-CR4 and R7-CR5provides a low speed drop out clamp for the converter output, preventingany output at speeds below a selected speed, so that low speed groundmovements such as taxiing and turning are not affected by the controlsystem.

The velocity comparator unit 109 comprises a differential op amp A4 thatis effective to compare the analog wheel speed voltage at the invertinginput terminal with the velocity reference voltage of the output signalof unit 113. The difference between the two signals is amplified andtransmitted as the comparator output signal. The output of thecomparator circuit drives the FEM and transient control loops, presentlyto be described, with a positive-going potential for a negativegoingwheel speed signal.

The circuit of the velocity reference unit 113 comprises an operationalintegrator whose output signal is equivalent to the instant velocity ofthe vehicle. The R13-CR9 net provides a unilateral feedback which forcesthe output signal of the velocity reference unit to track the wheelspeed voltage to initial condition at spin up of the wheel. The notednet is driven by the comparator output during spin up. the comparatoroutput is driven negatively at wheel spin up, which action draws currentout of the inverting input of A5, forcing the output positive until thetwo inputs to the comparator balance. When the input signals becomebalanced the comparator output returns to its quiescent level,preventing any further increase of the velocity reference voltage. Theresistor R12, by injecting more current into the input of op amp A5,aids in simulating aerodynamic drag.

The driving function or signal for the velocity reference, duringbraking is derived from the deceleration reference control unit 111, viathe connection represented by the heavy black line from the output of opamp A6 to the negative input terminal ofAS.

The output voltage of unit 113 in terms of input drive current isexpressed mathematically by in which V is output potential, C is thecapacitance of C7 and i is the input current. The input current isproportional to the drag force acting on the vehicle, and thecapacitance is an analog of aircraft mass.

The deceleration reference control unit 111 circuitry is a self-adaptivedrive for the velocity reference unit The output of the velocitycomparator unit 109 and representing the transient control signal, andthe output of the PBM control unit 117 and representing the normalcontrol signal, are applied as inputs to the summing op amp A8 of thesumming amplifier and transient control unit 115 for summation forcontrol of the valve driver unit 119. The PBM signal is applied toinverting input of A8 through resistor R29, and the tran- 113. Theoutput at junction J6 carrying the output of unit 111 is the derivativeof the slowly changing components of the wheel speed signal voltage,'thehigher frequency components being attenuated. Thus, since the slowlychanging component of the wheel speed sig nal is due to vehicledeceleration, the output signal of unit 111 is proportional to thevehicle decelerating drag forces.

The PBM control unit 117 is the principal controlling means foreffecting normal brake pressure correction. The driving input to theunit is the velocity error signal voltage from op amp A4 of the velocitycomparator unit 109, that signal being transmitted via,'heavy line lead04 (FIG. 3) from the output of A4 to the negative input signal terminalof op amp A7 of the PBM control unit. Op amp A7, in conjunction withcapacitor C15 and resistors R24, R25, provides an operational integratorthat integrates the velocity error signal from A4, to provide a smoothcontrol voltage. The divider network comprising resistors R26, R27 andR28 provides a velocity departure threshold for PBM control, bysummation of the signals at the inverting input terminal of op amp A7.Current flow through resistor R26 is away from the inverting input line;however the current from the velocity error signal line 04 throughresistor R24, R25 is toward the inverting input, and hence the twocurrents are subtractive.

sient control signal is applied to the non-inverting signal inputterminal of op amp A8 as shown. The transient control error signalthreshold is established by a divider network comprising resistors R32and R31 and rectifier CR19. The error signal threshold is such that thetransient control component does not affect the valve driver unit andcontrol valve until the velocity error output signal of comparator unit109 op amp A4 exceeds a level equivalent to a greater than normal rateof wheel deceleration. When that threshold value is exceeded, rectifierCR19 conducts and the conducted signal drives the non-inverting input ofop amp A8 through resistor R35. The PBM control signal is the timeintegral of the velocity error signal from the velocity comparator unit109, and the transient control signal is the velocity errorsignalamplified.

The output of the summing amplifier and transient control unit 1 15 issupplied to the valve driver unit 119. The latter unit comprises acontrol transistor Q1 and first and second current transistors Q2 and Q3whose currents are used to energize the coil of a servo valve such asthe primary valve of the previously described system. Maximum valvecurrent is clamped by a divider network including resistors R38 and R38and rectifier CR20. When the input voltage at the output of op amp A8increases to the point at which diode CR20 conducts, diode CR20 shutsoff, and base drive to transistor O1 is thus clamped to limit themaximum valve drive current.

In the circuitry shown, capacitor C17 connected to junction J6 (FIG. 3a)and to the non-inverting input terminal of op amp A8 is termed a leadcapacitor; and capacitor C17, connected between the input terminal andground is termed a lag capacitor.

In the event the wheel circuit for either wheel fails, a failuresignal(not shown) is energized in known manner. Thereupon the operator orpilot of the vehicle may optionally operate a switch S schematicallydepicted in FIG. 3b, whereby the valve units 121 for wheel 1 and 121 forwheel 2 are disconnected from their respective valve driver units 119and 119, respectively, and concurrently the output of op amp A8 of theremaining op.- erative wheel circuit is connected by one or the other ofdiodes CR21 and CR21 to the input terminal of a paired valve driver unit123 which is common to the two wheels of the pair and which unitcomprises a second valve device and connections thereto whereby thewheel circuit remaining active controls the brakerelieving of the brakesof both of the wheels.

In the valves 121 and 121', lapping of ports and passages is relied uponto provide some alleviation of hydraulic lag in the braking system.

The exemplary circuitry of FIGS. 3 3a and 3b comprises circuit elementsthe significant values or characteristics of which are made evident inthe follow-- ing Table II. An outstanding difference between themodified embodiment of the invention and that illustrated in FIGS. 1, 2aand 2b is that in the latter the velocity reference is produced fromsignals emanating from all of the wheel circuits whereas in the modifiedform each wheel circuit produces its own individual velocity reference.In the modified form the concept of the ratio of mass of vehicle to massof wheel is novel. Thus, as is indicated by the mathematical analysisherein, the values of certain circuit components depend upon the vehicleto wheel mass ratio.

TABLE II C1 0.0lmfd R' 38.3K ohms CRl lN645 C2 6.8mfd R1 1K ohms CR2 C36.8mfd R2 1K ohms CR3 C4 lOOpfd R3 2K ohms CR4 C5 ().lOrnfd R4 38.3Kohms CR5 C6 lpfd R 3.9K ohms CR6 C7 IOmfd R6 200K ohms CR7 C8 IOOpfd R71.5K ohms CR8 C9 0.47mfd R8 75K ohms CR9 C10 l00pfd R9 301K ohms CR10C11 IOmfd R10 51.1K ohms CRll C12 IOmfd R11 51K ohms CR12 C13 2.2mfd R12lmegohm CR13 C14 l00pfd R13 4.3K ohms CR14 C15 2.2mfd R14 K ohms CRISC16 IOOpfd R15 200K ohms CR16 C17 0.033mfd R16 20K ohms CR17 C170.022mfd C18 100pfd R17 200K ohms CR18 C19 lIOpfd R18 l87.3K ohms CR19A3 809GB R19 lmegohm CR19 A4 809CE R20 200K ohms CR20 A5 809CE R21 9.09Kohms CR2] A6 809CE R22 100K ohms CR21 A7 809CE R23 100K ohms CR22 A8809CE R23 1.24K ohms CR23 Q1 2 N930 R24 1.24K ohms CR24 Q2 2N2605 R25226K ohms CR25 Q3 2Nl893 R26 130K ohms CR26 R27 2.67K ohms CR27 R2824.3K ohms R29 127K ohms R30 562K ohms R31 10K ohms R32 2K ohms R33 93Kohms R34 49.9K ohms R35 178K ohms R36 150K ohms R37 45.3K ohms R38 3.65Kohms R38 4.22K ohms R39 1 10K ohms R41 3.0lK ohms R42 I 3.0lK ohms R43130K ohms R44 1K ohms R45 51.1K ohms R47 51.1K ohms R48 432ohms Whilethe particular braking control system for aircraft, vehicles and thelike, herein shown and disclosed in detail is fully capable of attainingthe objects and providing the advantages hereinbefore stated, it is tobe understood that it is merely illustrative of the presently preferredembodiments of the invention.

1 CLAIM:

1. In a braking control system for a vehicle having a plurality ofwheels having controlled brake application means, the combination foreach one of said wheels comprising:

analog means responsive to rotation of said one wheel for producing awheel-speed signal that is a function of the rotational speed of saidone wheel; simulator means supplied with information from saidwheel-speed signal for producing a variable reference signal simulatingvehicle speed without direct vehicle accelerative measurement, saidsimulator means utilizing wheel-speed information obtained solely fromsaid one wheel so as to avoid errorinducing introduction of wheel-speedinformation from other wheels;

comparator means supplied with said wheel-speed signal and with saidvariable reference signal for comparing said wheel-speed signal withsaid vari able reference signal to provide a slip error signal that is afunction of the difference between said wheel-speed signal and saidvariable reference signal; and

control means responsive to said slip error signal for producing a brakecontrol signal for modifying the action of said brake application means.

2. The invention defined in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum speed attained during wheel acceleration;

means for producing a force-analog signal indicative of the drag forcesacting on said vehicle; and

means for reducing said variable reference signal as a function of saidforce-analog signal.

3. The invention defined in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;

means for effecting decay of said variable reference signal at apredetermined rate; and

means responsive to said brake control signal for modifying the rate ofdecay of said variable reference signal.

4. The invention defined in claim 1 wherein said simulator meanscomprises:

means responsive to said wheel-speed signal to establish an initialvariable reference signal value derived from maximum wheel speedattained during wheel acceleration;

means responsive to said wheel-speed signal to produce a referencedeceleration signal representative of average vehicle deceleration; and

means for reducing said variable reference signal as a function of saidreference deceleration signal.

5. The inventiondefin ed in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;

means for producing a force-analog signal which varies as a function ofthe first derivative with respect to time of the low frequency contentof said wheel speed signal; and

means for reducing said variable reference signal as a function of saidforce-analog signal.

6. The invention defined in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;

means for producing a force-analog signal which varies as a function ofthe first derivative with respect to time of the low frequency contentof said wheel speed signal as indicative of the drag forces acting onsaid vehicle; and

means responsive to said force-analog signal for forming a time integralfunction of said forceanalog signal and reducing said variable referencesignal in accordance therewith.

7. The invention defined in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;

means for producing a force-analog signal which varies as a function ofthe first derivative with respect to the difference between saidwheel-speed signal and said variable reference signal; and

means for reducing said variable reference signal as a function of saidforce-analog signal.

8. The invention defined in claim 1 wherein said simulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;

means for producing a force-analog signal which varies as a function ofthe first derivative with respect to time of the difference between saidwheelspeed signal and said variable reference signal; and

means responsive to said force-analog signal for forming a time integralfunction of said forceanalog signal and reducing said variable referencesignal in accordance therewith.

9. The invention defined in claim 1 wherein said Sim-- ulator meanscomprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a force-analog signal which varies as a function ofthe first derivative with respect to time of the low frequency contentof the difference between said wheel-speed signal and said variablereference signal; and

means for reducing said variable reference signal as a function of saidforce-analog signal. 10. The invention defined in claim 1 wherein saidsimulator means comprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a force-analog signal which varies as a function ofthe'first derivative with respect to time of the low frequency contentof the difference between said wheel-speed signal and said variablereference signal; and I mass-analog means responsive to saidforce-analog signal for forming a time integral function of saidforce-analog signal in analog relation to vehicle mass and reducing saidvariable reference signal in accordance therewith.

11. The invention defined in claim 1 wherein said control meanscomprises:

means responsive to said error signal for producing a first brakecontrol signal which is a time integral function of said slip errorsignal and for producing a second brake control signal which is anonintegral function of said slip error signal; and

means responsive to said first and second brake control signals foreffecting said modification of the action of said brake applicationmeans.

12. The invention defined in claim 1 wherein said control means includesmeans for producing a brake control signal when said slip error signalexceeds a predetermined threshold level, said brake control signal beinga time integral function of both positive and negative variation of saidslip error signal from said threshold level.

13. In a braking control system for a vehicle having a plurality ofwheels having controlled brake application means, the combination foreach one of said wheels comprising:

analog means responsive to rotation of said one wheel for producing awheel-speed signal that is a function of the rotational speed of saidone wheel; reference generating means supplied with information fromsaid wheel-speed signal for producing a variable reference signalwithout direct vehicle accelerative measuremennsaid reference generatingmeans utilizing wheel-speed information obtained solely from said onewheel so as to avoid errorinducing introduction of wheel-speedinformation from other wheels; comparator means supplied with saidwheel-speed signal and with said variable reference signal for comparingsaid wheel-speed signal with said variable reference signal to providean error signal that is a function of the difference between saidwheelspeed signal and said variable reference signal; and control meansresponsive to said error signal for producing a brake control signal formodifying the action'of said brake application-means. 14. The inventiondefined in claim 13 wherein said reference generating means comprises: a

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a reference control signal which varies as a firstderivative function of said wheel speed signal; and meansfor reducingsaid variable reference signal as I a function of said reference controlsignal. 15. The invention defined in claim 13 wherein said referencegenerating means comprises:

means for establishing an initial variable reference value derived frommaximum wheel speed attained during wheel acceleration; means forproducing a reference control signal which varies as a low frequency,first derivative function of said wheel speed signal; and means forreducing said variable reference signal as a function of said referencecontrol signal. 16. The invention defined in claim .13 wherein saidreference generating means comprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a reference control signal which varies as a firstderivative function of said wheel speed signal; and means responsive tosaid reference control signal for forming a time integral function ofsaid reference control signal and reducing said variable referencesignal in accordance therewith. 17. The invention defined in claim 13wherein said reference generating means comprises:

means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a reference control signal which varies as a lowfrequency first derivative function of the said wheel speed signal; and

1. In a braking control system for a vehicle having a plurality ofwheels having controlled brake application means, the combination foreach one of said wheels comprising: analog means responsive to rotationof said one wheel for producing a wheel-speed signal that is a functionof the rotational speed of said one wheel; simulator means supplied withinformation from said wheel-speed signal for producing a variablereference signal simulating vehicle speed without direct vehicleaccelerative measurement, said simulator means utilizing wheel-speedinformation obtained solely from said one wheel so as to avoiderror-inducing introduction of wheel-speed information from otherwheels; comparator means supplied with said wheel-speed signal and withsaid variable reference signal for comparing said wheel-speed signalwiTh said variable reference signal to provide a slip error signal thatis a function of the difference between said wheel-speed signal and saidvariable reference signal; and control means responsive to said sliperror signal for producing a brake control signal for modifying theaction of said brake application means.
 2. The invention defined inclaim 1 wherein said simulator means comprises: means for establishingan initial variable reference signal value derived from maximum speedattained during wheel acceleration; means for producing a force-analogsignal indicative of the drag forces acting on said vehicle; and meansfor reducing said variable reference signal as a function of saidforce-analog signal.
 3. The invention defined in claim 1 wherein saidsimulator means comprises: means for establishing an initial variablereference signal value derived from maximum wheel speed attained duringwheel acceleration; means for effecting decay of said variable referencesignal at a predetermined rate; and means responsive to said brakecontrol signal for modifying the rate of decay of said variablereference signal.
 4. The invention defined in claim 1 wherein saidsimulator means comprises: means responsive to said wheel-speed signalto establish an initial variable reference signal value derived frommaximum wheel speed attained during wheel acceleration; means responsiveto said wheel-speed signal to produce a reference deceleration signalrepresentative of average vehicle deceleration; and means for reducingsaid variable reference signal as a function of said referencedeceleration signal.
 5. The invention defined in claim 1 wherein saidsimulator means comprises: means for establishing an initial variablereference signal value derived from maximum wheel speed attained duringwheel acceleration; means for producing a force-analog signal whichvaries as a function of the first derivative with respect to time of thelow frequency content of said wheel speed signal; and means for reducingsaid variable reference signal as a function of said force-analogsignal.
 6. The invention defined in claim 1 wherein said simulator meanscomprises: means for establishing an initial variable reference signalvalue derived from maximum wheel speed attained during wheelacceleration; means for producing a force-analog signal which varies asa function of the first derivative with respect to time of the lowfrequency content of said wheel speed signal as indicative of the dragforces acting on said vehicle; and means responsive to said force-analogsignal for forming a time integral function of said force-analog signaland reducing said variable reference signal in accordance therewith. 7.The invention defined in claim 1 wherein said simulator means comprises:means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a force-analog signal which varies as a function ofthe first derivative with respect to the difference between saidwheel-speed signal and said variable reference signal; and means forreducing said variable reference signal as a function of saidforce-analog signal.
 8. The invention defined in claim 1 wherein saidsimulator means comprises: means for establishing an initial variablereference signal value derived from maximum wheel speed attained duringwheel acceleration; means for producing a force-analog signal whichvaries as a function of the first derivative with respect to time of thedifference between said wheel-speed signal and said variable referencesignal; and means responsive to said force-analog signal for forming atime integral function of said force-analog signal and reducing saidvariable reference signal in accordance therewith.
 9. The inventiondefined in claim 1 wherein said simulator means comprises: means forestabLishing an initial variable reference signal value derived frommaximum wheel speed attained during wheel acceleration; means forproducing a force-analog signal which varies as a function of the firstderivative with respect to time of the low frequency content of thedifference between said wheel-speed signal and said variable referencesignal; and means for reducing said variable reference signal as afunction of said force-analog signal.
 10. The invention defined in claim1 wherein said simulator means comprises: means for establishing aninitial variable reference signal value derived from maximum wheel speedattained during wheel acceleration; means for producing a force-analogsignal which varies as a function of the first derivative with respectto time of the low frequency content of the difference between saidwheel-speed signal and said variable reference signal; and mass-analogmeans responsive to said force-analog signal for forming a time integralfunction of said force-analog signal in analog relation to vehicle massand reducing said variable reference signal in accordance therewith. 11.The invention defined in claim 1 wherein said control means comprises:means responsive to said error signal for producing a first brakecontrol signal which is a time integral function of said slip errorsignal and for producing a second brake control signal which is anon-integral function of said slip error signal; and means responsive tosaid first and second brake control signals for effecting saidmodification of the action of said brake application means.
 12. Theinvention defined in claim 1 wherein said control means includes meansfor producing a brake control signal when said slip error signal exceedsa predetermined threshold level, said brake control signal being a timeintegral function of both positive and negative variation of said sliperror signal from said threshold level.
 13. In a braking control systemfor a vehicle having a plurality of wheels having controlled brakeapplication means, the combination for each one of said wheelscomprising: analog means responsive to rotation of said one wheel forproducing a wheel-speed signal that is a function of the rotationalspeed of said one wheel; reference generating means supplied withinformation from said wheel-speed signal for producing a variablereference signal without direct vehicle accelerative measurement, saidreference generating means utilizing wheel-speed information obtainedsolely from said one wheel so as to avoid error-inducing introduction ofwheel-speed information from other wheels; comparator means suppliedwith said wheel-speed signal and with said variable reference signal forcomparing said wheel-speed signal with said variable reference signal toprovide an error signal that is a function of the difference betweensaid wheel-speed signal and said variable reference signal; and controlmeans responsive to said error signal for producing a brake controlsignal for modifying the action of said brake application means.
 14. Theinvention defined in claim 13 wherein said reference generating meanscomprises: means for establishing an initial variable reference signalvalue derived from maximum wheel speed attained during wheelacceleration; means for producing a reference control signal whichvaries as a first derivative function of said wheel speed signal; andmeans for reducing said variable reference signal as a function of saidreference control signal.
 15. The invention defined in claim 13 whereinsaid reference generating means comprises: means for establishing aninitial variable reference value derived from maximum wheel speedattained during wheel acceleration; means for producing a referencecontrol signal which varies as a low frequency first derivative functionof said wheel speed signal; and means for reducing said variablereference signal as a function of said reference control signal.
 16. Theinvention defined in claim 13 wherein said reference generating meanscomprises: means for establishing an initial variable reference signalvalue derived from maximum wheel speed attained during wheelacceleration; means for producing a reference control signal whichvaries as a first derivative function of said wheel speed signal; andmeans responsive to said reference control signal for forming a timeintegral function of said reference control signal and reducing saidvariable reference signal in accordance therewith.
 17. The inventiondefined in claim 13 wherein said reference generating means comprises:means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a reference control signal which varies as a lowfrequency first derivative function of the said wheel speed signal; andmeans responsive to said reference control signal for forming a timeintegral function of said reference control signal and reducing saidvariable reference signal in accordance therewith.
 18. The inventiondefined in claim 13 wherein said reference generating means comprises:means for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration;means for producing a reference control signal which varies as a firstderivative function of the difference between said wheel-speed signaland said variable reference signal; and means for reducing said variablereference signal as a function of said reference control signal.
 19. Theinvention defined in claim 13 wherein said reference generating meanscomprises: means for establishing an initial variable reference signalvalue derived from maximum wheel speed attained during wheelacceleration; means for producing a reference control signal whichvaries as a first derivative function of the difference between saidwheel-speed signal and said variable reference signal; and meansresponsive to said reference control signal for forming a time integralfunction of said reference control signal and reducing said variablereference signal in accordance therewith.
 20. The invention defined inclaim 13 wherein said reference generating means comprises: means forestablishing an initial variable reference signal value derived frommaximum wheel speed attained during wheel acceleration; means forproducing a reference control signal which varies as a low frequencyfirst derivative function of the difference between said wheel-speedsignal and said variable reference signal; and means for reducing saidvariable reference signal as a function of said reference controlsignal.
 21. The invention defined in claim 13 wherein said referencegenerating means comprises: means for establishing an initial variablereference signal value derived from maximum wheel speed attained duringwheel acceleration; means for producing a reference control signal whichvaries as a low frequency first derivative function of the differencebetween said wheel-speed signal and said variable reference signal; andmeans responsive to said reference control signal for forming a timeintegral function of said reference control signal and reducing saidvariable reference signal in accordance therewith.
 22. The inventiondefined in claim 13 wherein said control means comprises: meansresponsive to said error signal for producing a first brake controlsignal which is a time integral function of said error signal and forproducing a second brake control signal which is a non-integral functionof said error signal; and means responsive to said first and secondbrake control signals for effecting said modification of the action ofsaid brake application means.
 23. The invention defined in claim 13wherein said control means includes means for producing a brake controlsignal when said error siGnal exceeds a predetermined threshold level,said brake control signal being a time integral function of bothpositive and negative variation of said error signal from said thresholdlevel.
 24. A system for modifying the action of the brake applicationmeans for a wheel of a vehicle, comprising: analog means responsive torotation of said wheel for producing a wheel-speed signal that is afunction of the rotational speed of said wheel; simulator means suppliedwith information from said wheel-speed signal for producing a variablereference signal simulating vehicle speed without direct vehicleaccelerative measurement, said simulator means comprising means forestablishing an initial variable reference signal value derived frommaximum wheel speed attained during wheel acceleration, means forproducing a force-analog signal which varies as a function of the firstderivative with respect to time of the low frequency content of saidwheel speed signal, and means for reducing said variable referencesignal as a function of said force-analog signal; comparator meanssupplied with said wheel-speed signal and with said variable referencesignal for comparing said wheel-speed signal with said variablereference signal to provide a slip error signal that is a function ofthe difference between said wheel-speed signal and said variablereference signal; and control means responsive to said slip error signalfor producing a brake control signal for modifying the action of saidbrake application means.
 25. The invention defined in claim 24 whereinsaid means for reducing said variable reference signal as a function ofsaid force analog signal comprises means responsive to said force analogsignal for forming a time integral function of said force-analog signaland reducing said variable reference signal in accordance therewith. 26.A braking system for modifying the action of the brake application meansfor a wheel of a vehicle comprising: analog means responsive to rotationof said wheel for producing a wheel-speed signal that is a function ofthe rotational speed of one wheel; simulator means supplied withinformation from said wheel-speed signal for producing a variablereference signal simulating vehicle speed without direct vehicleaccelerative measurement, said simulator means comprising means forestablishing an initial variable reference signal value derived frommaximum wheel speed attained during wheel acceleration, means forproducing a force-analog signal which varies as a function of the firstderivative with respect to time of the difference between saidwheel-speed signal and said variable reference signal, and means forreducing said variable reference signal as a function of saidforce-analog signal; comparator means supplied with said wheel-speedsignal and with said variable reference signal for comparing saidwheel-speed signal with said variable reference signal to provide a sliperror signal that is a function of the difference between saidwheel-speed signal and said variable reference signal; and control meansresponsive to said slip error signal for producing a brake controlsignal for modifying the action of said brake application means.
 27. Theinvention defined in claim 26 wherein said means for reducing saidvariable reference signal as a function of said force analog signalcomprises means responsive to said force analog signal for forming atime integral function of said force-analog signal and reducing saidforce analog signal in accordance therewith.
 28. A system for modifyingthe action of the brake application means for a wheel of a vehiclecomprising: analog means responsive to rotation of said wheel forproducing a wheel-speed signal that is a function of the rotationalspeed of said wheel; simulator means supplied with information from saidwheel-speed signal for producing a variable reference signal simulatingvehicle speed, said simulator means comprising means for establishing anInitial variable reference signal value derived from maximum wheel speedattained during wheel acceleration, means for producing a force-analogsignal which varies as a function of the first derivative with respectto time of the low frequency content of the difference between saidwheel-speed signal and said variable reference signal, and means forreducing said variable reference signal as a function of saidforce-analog signal; comparator means supplied with said wheel-speedsignal and with said variable reference signal for comparing saidwheel-speed signal with said variable reference signal to provide a sliperror signal that is a function of the difference between saidwheel-speed signal and said variable reference signal; and control meansresponsive to said slip error signal for producing a brake controlsignal for modifying the action of said brake application means.
 29. Theinvention defined in claim 28 wherein said means for reducing saidvariable reference signal as a function of said force analog signalcomprises mass-analog means responsive to said force analog signal forforming a time integral function of said force-analog signal in analogrelation to vehicle means and reducing said variable reference signal inaccordance therewith.
 30. The invention defined in claim 29 wherein saidcontrol means comprises: means responsive to said error signal forproducing a first brake control signal which is a time integral functionof said slip error signal and for producing a second brake controlsignal which is a non-integral function of said slip error signal; andmeans responsive to said first and second brake control signals foreffecting said modification of the action of said brake applicationmeans.
 31. The invention defined in claim 29 wherein said control meansincludes means for producing a brake control signal when said slip errorsignal exceeds a predetermined threshold level, said brake controlsignal being a time integral function of both positive and negativevariation of said slip error signal from said threshold level.
 32. Asystem for modifying the action of the brake application means for awheel of a vehicle, comprising: analog means responsive to rotation ofsaid wheel for producing a wheel-speed signal that is a function of theinstantaneous rotational speed of said wheel; simulator means suppliedwith information from said wheel-speed signal for producing a variablereference signal simulating vehicle speed, said simulator meanscomprising means for establishing an initial variable reference signalvalue derived from maximum wheel speed attained during wheelacceleration, means for producing a force-analog signal which varies asa function of the first derivative with respect to time of the lowfrequency content of the difference between said variable referencesignal and said wheel-speed signal, and mass-analog means responsive tosaid force-analog signal for forming a time integral function of saidforce-analog signal in analog relation to vehicle mass and reducing saidvariable reference signal in accordance therewith; comparator meanssupplied with said wheel-speed signal and with said variable referencesignal for comparing said wheel-speed signal with said variablereference signal to provide a slip error signal that is a function ofthe difference between said wheel-speed signal and said variablereference signal; means responsive to said slip error signal forproducing a first brake control signal when said slip error signalexceeds a first predetermined threshold level, said first brake controlsignal being a time integral function of both positive and negativevariation of said slip error signal from said first threshold level;means responsive to said error signal for producing a second brakecontrol signal when said slip error signal exceeds a secondpredetermined threshold level, said second brake control signal being anon-integral function of said slip error signal; and Means for summingsaid first and second brake control signals to produce a composite brakecontrol signal for modifying the action of said brake application means.33. The invention defined in claim 32 wherein said second brake controlsignal is substantially a linear function of said slip error signal. 34.The invention defined in claim 32 further comprising capacitor means forproviding a system lag compensating signal for summation with said firstand second brake control signals to form said composite brake controlsignal.
 35. A system for modifying the action of the brake applicationmeans for a wheel of a vehicle, comprising: analog means responsive torotation of said wheel for producing a wheel-speed signal that is afunction of the instantaneous rotational speed of said wheel; referencegenerating means supplied with information from said wheel-speed signalfor producing a variable reference signal without direct vehicleaccelerative measurement, said reference generating means comprisingmeans for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration,means for producing a reference control signal which varies as a firstderivative function of said wheel speed signal, and means for reducingsaid variable reference signal as a function of said reference controlsignal; comparator means supplied with said wheel-speed signal and withsaid variable reference signal for comparing said wheel-speed signalwith said variable reference signal to provide an error signal that is afunction of the difference between said wheel-speed signal and saidvariable reference signal; and control means responsive to said errorsignal for producing a brake control signal for modifying the action ofsaid brake application means.
 36. The invention defined in claim 35wherein said means for reducing said variable reference signal as afunction of said reference control signal comprises means responsive tosaid reference control signal for forming a time integral function ofsaid reference control signal in accordance therewith.
 37. A system formodifying the action of the brake application means for a wheel of avehicle comprising: analog means responsive to rotation of said wheelfor producing a wheel-speed signal that is a function of the rotationalspeed of said wheel; reference generating means supplied withinformation from said wheel-speed signal for producing a variablereference signal without direct vehicle accelerative measurement, saidreference generating means comprising means for establishing an initialvariable reference value derived from maximum wheel speed attainedduring wheel acceleration, means for producing a reference controlsignal which varies as a low frequency first derivative function of saidwheel speed signal, and means for reducing said variable referencesignal as a function of said reference control signal; comparator meanssupplied with said wheel-speed signal and with said variable referencesignal for comparing said wheel-speed signal with said variablereference signal to provide an error signal that is a function of thedifference between said wheel-speed signal and said variable referencesignal; and control means responsive to said error signal for producinga brake control signal for modifying the action of said brakeapplication means.
 38. The invention defined in claim 37 wherein saidmeans for reducing said variable reference signal as a function of saidreference control signal comprises means responsive to said referencecontrol signal for forming a time integral function of said referencecontrol signal and reducing said variable reference signal in accordancetherewith.
 39. A system for modifying the action of the brakeapplication means for a wheel of a vehicle comprising: analog meansresponsive to rotation of said wheel for producing a wheel-speed signalthat is a function of the rotational speed of said whEel; referencegenerative means supplied with information from said wheel-speed signalfor producing a variable reference signal without direct vehicleaccelerative measurement, said reference generating means comprisingmeans for establishing an initial variable reference signal valuederived from maximum wheel speed attained during wheel acceleration,means for producing a reference control signal which varies as a firstderivative function of the difference between said wheel-speed signaland said variable reference signal, and means for reducing said variablereference signal as a function of said reference control signal;comparator means supplied with said wheel-speed signal and with saidvariable reference signal for comparing said wheel-speed signal withsaid variable reference signal to provide an error signal that is afunction of the difference between said wheel-speed signal and saidvariable reference signal; and control means responsive to said errorsignal for producing a brake control signal for modifying the action ofsaid brake application means.
 40. The invention defined in claim 39wherein said means for reducing said variable reference signal as afunction of said reference control signal comprises means responsive tosaid reference control signal for forming a time integral function ofsaid reference control signal and reducing said variable referencesignal in accordance therewith.
 41. A system for modifying the action ofthe brake application means for a wheel of a vehicle, comprising: analogmeans responsive to rotation of said wheel for producing a wheel-speedsignal that is a function of the rotational speed of said wheel;reference generating means supplied with information from saidwheel-speed signal for producing a variable reference signal, saidreference generating means comprising means for establishing an initialvariable reference signal without direct vehicle accelerativemeasurement value derived from maximum wheel speed attained during wheelacceleration, means for producing a reference control signal whichvaries as a low frequency first derivative function of the differencebetween said wheel-speed signal and said variable reference signal, andmeans for reducing said variable reference signal as a function of saidreference control signal; comparator means supplied with saidwheel-speed signal and with said variable reference signal for comparingsaid wheel-speed signal with said variable reference signal to providean error signal that is a function of the difference between saidwheel-speed signal and said variable reference signal; and control meansresponsive to said error signal for producing a brake control signal formodifying the action of said brake application means.
 42. The inventiondefined in claim 41 wherein said means for reducing said variablereference signal as a function of said reference control signalcomprises means responsive to said reference control signal for forminga time integral function of said reference control signal and reducingsaid variable reference signal in accordance therewith.
 43. Theinvention defined in claim 41 wherein said control means comprises:means responsive to said error signal for producing a first brakecontrol signal which is a time integral function of said error signaland for producing a second brake control signal which is a non-integralfunction of said error signal; and means responsive to said first andsecond brake control signals for effecting said modification of theaction of said brake application means.
 44. The invention defined inclaim 41 wherein said control means includes means for producing a brakecontrol signal when said error signal exceeds a predetermined thresholdlevel, said brake control signal being a time integral function of bothpositive and negative variation of said error signal from said thresholdlevel.
 45. A system for modifying the action of the brake applicationmeans for a wheel of a vehicle, comprising: analog means responsive torotation of said wheel for producing a wheel-speed signal that is afunction of the instantaneous rotational speed of said wheel; referencegenerating means supplied with information from said wheel-speed signalfor producing a variable reference signal, said reference generatingmeans comprising means for establishing an initial variable referencesignal value derived from maximum wheel speed attained during wheelacceleration, means for producing a reference control signal whichvaries as a low frequency first derivative function of the differencebetween said wheel-speed signal and said variable reference signal,means responsive to said reference control signal for forming a timeintegral function of said reference control signal and reducing saidvariable reference signal in accordance therewith; comparator meanssupplied with said wheel-speed signal and with said variable referencesignal for comparing said wheel-speed signal with said variablereference signal to provide an error signal that is a function of thedifference between said wheel-speed signal and said variable referencesignal; means responsive to said error signal for producing a firstbrake control signal when said error signal exceeds a firstpredetermined threshold level, said first brake control signal being atime integral function of both positive and negative variation of saiderror signal from said first threshold level; means responsive to saiderror signal for producing a second brake control signal when said errorsignal exceeds a second predetermined threshold level, said second brakecontrol signal being a non-integral function of said error signal; andmeans for summing said first and second brake control signals to producea composite brake control signal for modifying the action of said brakeapplication means.
 46. The invention defined in claim 45 wherein saidsecond brake control signal is substantially a linear function of saiderror signal.
 47. The invention defined in claim 45 further comprisingcapacitor means for providing a system lag compensating signal forsummation with said first and second brake control signals to form saidcomposite brake control signal.
 48. In a braking control system for avehicle having a plurality of wheels having controlled brake applicationmeans, the combination for each one of said wheels comprising: analogmeans responsive to rotation of said one wheel for producing awheel-speed signal that is a function of the instantaneous rotationalspeed of said one wheel; reference generating means supplied withinformation from said wheel-speed signal for producing a variablereference signal without direct vehicle accelerative measurement, saidreference generating means utilizing wheel-speed information obtainedsolely from said one wheel so as to avoid error-inducing introduction ofwheel-speed information from other wheels; comparator means suppliedwith said wheel-speed signal and with said variable reference signal forcomparing said wheel-speed signal with said variable reference signal toprovide an error signal that is a function of the difference betweensaid wheel-speed signal and said variable reference signal; controlmeans responsive to said error signal for producing a brake controlsignal for modifying the action of said brake application means; andsaid reference generating means comprising means for establishing aninitial variable reference signal value derived from maximum wheel speedattained during wheel acceleration, means for effecting decay of saidvariable reference signal at a predetermined rate, and means responsiveto the brake control signals for all of said braked wheels for modifyingthe rate of decay of said variable reference signal.